1. Field of the Invention
The present invention relates to a synchronous motor which can be used as a drive motor of an industrial machine and a driving motor of a vehicle, particularly a hybrid vehicle.
2. Description of Related Art
As a compact and high output motor, there is known the so-called IPM motor (Interior Permanent Magnet Motor) which uses reluctance torque derived from attraction force of its core in addition to magnet torque. Generally, the IPM motor achieves a high output density by generating a large magnet torque using high-performance magnets or a large amount of magnets, and utilizing the reluctance torque accessorily. Meanwhile, when the IPM motor is used as a driving motor of a vehicle, it is required to be low in manufacturing cost, in addition to having a high output density. As a motor satisfying such requirements, there is known the so-called auxiliary magnet assisted reluctance motor which can generate a high reluctance torque using a small amount of magnets by ingenuity in the shape of its core, in contrast to conventional IPM motors which are designed giving great importance to the magnet torque. For example, refer to Japanese Patent Application Laid-open No. 2008-295282.
Generally, the reluctance torque T is given by the following equation (1).
T=(Lq−Ld)×Id·Iq . . . (1) where Lq is a q-axis inductance, Iq is a q-axis current, Ld is a d-axis inductance, and Id is a d-axis current.
The equation (1) shows that the reluctance torque increases with the increase of the inductance difference between the d-axis and the q-axis which are perpendicular to each other. The above patent document describes that the d-axis inductance Ld is made smaller by providing a large concave portion (magnetic gap) in the d-axis to increase the inductance difference as much as possible. However, the motor described in the above patent document has a problem as described in the following.
The d-axis inductance Ld is very small, and accordingly most of the magnetic flux passes through the q-axis. Accordingly, the ratio of the cross-sectional area of the magnetic path of the rotor (that is, the product of the circumferential width of one stator tooth, the number of teeth for one stator pole and the lamination thickness of the core) to the cross-sectional area of the q-axis flux passage of the rotor (that is, the product of the circumferential width of the q-axis for one rotor pole and the lamination thickness of the core) is large. Hence, when the stator winding is excited, there occurs a lot of inefficient flux which not only does not contribute to the output torque, but also causes iron loss in the stator core.